Always verify the following before running any program:
If you can't confidently answer "yes," go back and check. Accidentally running the wrong program with the wrong setup could cause a serious crash.
Many tools are alike. Always double check that tool number matches what the program asks for.
If it can wiggle it free, don't machine it.
This is what stands between you and a hungry spindle that flings sharp chips (and in the event of a crash, parts) in your direction.
Also, did you read The Emergency Stop Button? If you haven't, go do that first.